Oleaginous materials such as crude oils, lubricating oils, heating oils and other distillate petroleum fuels, for example diesel fuels, contain alkanes that at low temperature tend to precipitate as large crystals of wax forming a gel structure so that the fuel or oil loses its ability to flow. The lowest temperature at which the crude oil, lubricating oil or fuel oil will still flow is known as the pour point. In the case of fuels as the temperature of the fuel falls and approaches the pour point, difficulties arise in transporting the fuel through lines and pumps. Further, the wax crystals tend to plug fuel lines, screens and filters at temperatures above the pour point. These problems are well recognized in the art, and various additives have been proposed, many of which are in commercial use, for depressing the pour point of fuel oils. Similarly, other additives have been proposed and are in commercial use, for reducing the size and changing the shape of the wax crystals that do form. Other additives may also retain wax crystals in suspension, and may be referred to as anti-settling aids. Additives may also be added to improve other properties of the fuel oil, for example to act as corrosion inhibitors, or detergents or to inhibit sediment formation.
The invention is relevant but not restricted to fuel oils, including those boiling in the gasoline range, but is particularly relevant to those liquids referred to as middle distillate fuel oils. These fuel oils typically boil in the range of about 120.degree. C. to about 500.degree. C., and may comprise atmospheric distillate or vacuum distillate, or cracked gas oil, or a mixture of straight-run and cracked distillates. The most common petroleum distillate fuel oils are kerosene, jet fuels, diesel fuels, and heating oils. In any event it is almost always necessary to add a small proportion, for example between 10 and 2,000 ppm by weight, of additives to the liquid as produced by a refinery, in order to produce a fuel or oil which is suitable for sale and meets desired specifications. Typically a refiner would use one additive composition for all fuels or might, in some cases, use one additive composition (A) if producing diesel fuel, or a different additive composition (B) if producing heating oil; each additive composition (A or B) comprising a mixture of the chemically-different types of additive discussed above, chosen to ensure the desired specification is met. Where the characteristics of the untreated fuel oil vary (due for example to changes in refinery operation or changes in crude oil), the refiner ensures that the desired specification continues to be met by adjusting the proportion of the additive composition (say A) which is added. In the same way the refiner can produce fuel oils which meet different specifications, for example for use in different climates, by adjusting the proportion of the additive composition (say A) which is added.